The present invention relates to a star sensor for use in the attitude control of satellites and, more particularly, to a star sensor using a charge-coupled device (CCD) in its star imaging array.
In one of the conventional techniques of using a CCD, the output of the CCD, after being amplified, is applied to a sample-and-hold circuit for voltage retention. The sustained voltage is then compared with a reference voltage by a voltage comparator. This reference voltage corresponds to the star magnitude, and the sustained voltage, if found to be above the reference voltage, is subjected to analogdigital conversion and supplied to a dataprocessing circuit. At the same time, the picture element (pel) position information (i.e., the coordinates of the star) at this moment is supplied from a coordinate counter, which counts the clock pulses outputted from the CCD driving circuit, to the data processing circuit. The latter circuit stores in a buffer memory the CCD output voltage thereby obtained, which is not below the reference voltage, and its positional information with respect to every pel on the CCD. Then, its picks out the highest among the stored voltages and determines the central position of the star on that basis.
Since all the data regarding the pels which exceed the reference value are stored, the conventional star sensor requires a large-capacity buffer memory, which is costly to manufacture and results in added weight. Furthermore, background light rays entering uniformly into the optical system tend to be sensed and affect star image signals adversely, depending on how high the reference voltage of the voltage comparator is set.
Star sensors using CCDs are described in G. Borghi et al, "STAR DETECTION AND TRACKING USING CCDs", IFAC Automatic Control in Space, 1982, pp. 263 to 269. The proposed star sensor has a star search mode and a star track mode, and has a great advantage in accuracy, but it does have the following problems: it is complex in circuitry because it requires two sets of hardware, one for the search mode and the other for the track mode, and also involves the need for different software arrangements for the two modes. Furthermore, this paper in no way touches on the resolution of background light.